The following studies were carried out or are now being undertaken to determine what role, if any, cyclic nucleotides play in the regulation of the mammalian cell division cycle: 1) The effects of added cyclic nucleotides and its derivatives upon HeLa cell growth. 2) Determination of the levels of cAMP and the effect of catechol amines on these levels. The effect of cell density upon these parameters. 3) Determination of changes in cyclic AMP concentrations as cells progress through a synchronized cell cycle. 4) The effect upon growth of induced changes in endogenous cAMP levels. 5) Correlation of changes in cyclic AMP with ratios of active : total cyclic AMP dependent protein kinase. 6) Comparisons of above parameters between HeLa cells and mouse lymphoma cells.